In particular, relatively large photovoltaic (PV) systems (photovoltaic is referred to below as PV) can have a PV generator which often comprises a plurality of, but at least two, PV subgenerators connected in parallel. Such a PV subgenerator can be a so-called string, for example, which is formed from a plurality of PV modules connected in series. The PV generator is connected to one or more inverters within the PV system, which inverters convert the direct current (DC) produced by the PV generator into an alternating current (AC) suitable for feeding a power supply system. In such PV systems, isolation between the PV generator and the inverter is generally required. Depending on requirements, this isolation can be a functional disconnection simply via a semiconductor switch, galvanic isolation or even galvanic isolation at all poles. In the case of galvanic isolation or galvanic isolation at all poles, this can be implemented with the aid of electromechanical switches. In this case, a dedicated electromechanical switch is often associated with each of the PV subgenerators in order to be able to selectively disconnect the PV subgenerators, for example in the event of a fault. An electromechanical switch is understood to mean an electromechanically actuated switch, actuated by an electric motor or an electromagnet, for example. Electromagnet-actuated switches are also referred to as relays or contactors.
In PV systems, owing to the high DC voltage and the high prevailing direct currents, arcs represent a problem which should not be underestimated and are one of the primary causes of fires. If an arc is identified, means for quenching the arc in the respective PV subgenerators should be activated as quickly as possible. Such means represent, for example, the mentioned switches for isolating the PV subgenerators from the input of the inverter. If a PV subgenerator is isolated from the inverter, the current through an arc which is connected in series with the PV subgenerator is reduced, as a result of which this arc is quenched. Document DE 10 2011 000 737 A1 discloses, for example, an isolating switch which is activated automatically after detection of an arc as part of a protection device, which is, for example, arranged between a PV generator and an inverter, close to the PV generator.
In particular, in the case of large PV systems, simultaneous decoupling of the entire PV generator, i.e. simultaneous decoupling of all of the PV subgenerators, is disadvantageous since instabilities in the power supply system to which the inverter feeds can occur. In addition, subsequent faultfinding is complex since, in the case of such a simultaneous disconnection of all of the PV subgenerators, it is not possible for the arc to be localized. If, on the other hand, the PV subgenerators are isolated from the DC input circuit of the inverter successively, the location of the arc can be restricted at least to the level of the PV subgenerators. A similar method for localizing an arc is described in document DE 101 55 795 C1 in connection with a motor vehicle power supply system, which comprises a plurality of power supply subsystems.
Owing to the inertia of the electromechanical switches via which the PV subgenerators are connected to the inverter, a single switching operation has a minimum switching duration, however, which can be in the region of a few tens of milliseconds. Added to this is in each case the time which is required for establishing, after a switching operation, whether the previously detected arc still exists or has been quenched. In the case of large PV systems with a correspondingly large number of PV subgenerators, sequential or successive disconnection can take a few seconds under certain circumstances. In the worst case, the arc can be present up until disconnection of the last subgenerator, i.e. throughout the entire time of the switching sequences. This is undesirable for safety reasons, for example owing to the risk of a fire originating from an arc, and, under certain circumstances, for example in accordance with the US standard UL1699B, is also impermissible.